The prior art has concerned itself, for many years, with the problem of reducing or eliminating the loss of water in galvanic cells using aqueous electrolyte, and also with avoiding build up of excessive gas pressure in sealed cells. Hydrogen gas is evolved during charge or standby by several electrode materials such as aluminum, magnesium, zinc, iron, lead, etc. The electrodes in general do not have the capability of recombining the hydrogen and the evolved gas is usually vented, causing water loss, or pressure build up in hermetically sealed cells. In sealed cells, depending on the amount of hydrogen present and the rate of generation, excessive gas pressure can build up causing rupture of the safety vent and loss of electrolyte--resulting in cell failure and electrolyte leakage. It has previously been found that cells having a porous manganese dioxide cathode have the capability of recombining the hydrogen, provided catalytically active materials are applied to the cathode.
Two approaches are often used in efforts to solve the problems. These are:
1. Catalytic recombination of hydrogen and oxygen inside or outside the battery; in the latter case, provisions are made for the return of the product water to the electrolyte chamber [U.S. Pat. No. 3,630,778 (1971), U.S. Pat. No. 3,598,653 (1971), U.S. Pat. No. 3,622,398 (1971), U.S. Pat. No. 3,701,691 (1972)]. PA0 2. Use of an auxiliary (third) electrode as an overcharge recombination reactor as described in "Electrochem. Technol., 4, 383 (1966) by P. Ruetschi and J. B. Ockerman.
Kozawa et al in Electrochemica Acta Volume 26, No. 10 at pages 1489 to 1493, published in 1981, discussed the use of silver-catalyzed manganese dioxide as a hydrogen absorber. There, considerable studies were made by mixing various ranges of AgO or Ag.sub.2 O with electrolytic manganese dioxide (EMD), and in some instances by mixing EMD with AgNO.sub.3 solution. It was found, however, that at silver concentrations below about 0.3% of the EMD content, the recombination rate of the silver-catalyzed MnO2 was essentially no different than that of uncatalyzed EMD.
Indeed, those same two authors in KORDESCH et al U.S. Pat. No. 4,224,384 report excellent hydrogen gas recombination capability of dry MnO.sub.2 powder catalyzed with salts or oxides of platinum, palladium, ruthenium, rhodium, arsenic and lead. These materials, however, when employed in a wetted MnO.sub.2 matrix, did not show significant hydrogen recombination rates at near atmospheric pressures. It has now surprisingly been found that these materials do exhibit hydrogen recombination properties when at least partially wetted by electrolyte, and in the pressure range of from substantially zero gauge pressure up to the relief pressure of the cell.
According to the invention there is provided a primary or rechargeable electrochemical sealed cell in which hydrogen may evolve, having a manganese dioxide cathode, a zinc anode, and an aqueous electrolyte (which may be alkaline, or ammonium chloride or zinc chloride, or mixtures thereof) contacting the anode and the cathode. There is a further auxiliary cathode material provided comprising a catalyst (which may be deposited on a porous substrate) for the recombination of pressurized hydrogen with the manganese dioxide, the auxiliary cathode material being located so as to be at least partially wetted by the electrolyte. The auxiliary cathode material may comprise a discrete element located in the cell, or it may be distributed throughout the cathode.
The substrate, when used, may be carbon or graphite, and the catalyst may be carbon, catalytically active noble or other metals, their salts and their oxides. The metals may be iron, zirconium, yttrium, calcium, magnesium, copper, lead, nickel, titanium, lanthanum, chromium, vanadium, tantalum, and catalytically active alloys thereof; as well as AB.sub.5, "Mischmetal", or nonstoichiometric type alloys which can store hydrogen gas in their interior lattices. The noble metals, which may be mixed with carbon, may be, for example, platinum, palladium, ruthenium, rhodium or silver, or their salts or their oxides.
The auxiliary cathode material may be provided either in admixture with the manganese dioxide cathode, or as a discrete auxiliary electrode. In either event, it is in electronic contact with the cathode, and with no substantial electrical resistance between them. When a cathode comprises a plurality of pellets, the auxiliary cathode material may be in admixture with only one or all of the pellets; and if the cathode is extruded as a single sleeve, the auxiliary cathode material may be distributed throughout the cathode.
When the auxiliary cathode material is provided as an auxiliary discrete electrode, and the manganese dioxide cathode is cylindrically located about an anode core, then the auxiliary electrode may suitably be an annulus or ring of similar diameters to the cathode and located in electronic contact with it at one end of the cathode, or between pellets.
The present invention may provide economic and effective means of recombining hydrogen gas in galvanic cells. Noble metals such as platinum, palladium, rhodium, iridium, ruthenium, and osmium show high catalytic activity for hydrogen oxidation. In alkaline electrolytes, nickel and alloys of nickel with other metals (e.g. titanium and lanthanum) were found to be active catalysts.
If an annulus is used as an auxiliary catalysed electrode, then conveniently the auxiliary electrode may be a gas diffusion electrode. Gas diffusion electrodes that may be particularly applicable to the present invention are described in the co-pending United States Patent Disclosure "Metal and Metal Oxide Catalyzed Electrodes for Electrochemical Cells, and Method of Making Same" by K. Tomantschger and K. Kordesch, Ser. No. 234,933, filed Aug. 22, 1988, and can be employed if higher recombination current densities are desired.
Embodiments of the invention will now be described by way of illustration with reference to the drawings in conjunction with the Examples, describing various electrodes of the invention and their operating characteristics.